CIRED2005 Session No 3
REACTIVE POWER COMPENSATION AND VOLTAGE CONTROL  IN JINAN POWER DISTRIBUTION SYSTEM
Yutian LIU      Jiachuan SHI                                                    Xia QIAN
School of Electrical Engineering, Shandong University              Jinan Electric Power Company                                                      Jinan 250061, China                                                Jinan 250012, China
liuyt@sdu.edu                                                    qianx@hdpi
INTRODUCTION
The main reasons of voltage and reactive power problems in Jinan power distribution system are unreasonable network structure, large load change ranges, lack of reactive power compensation and voltage regulation means. Based on engineering practice and current technology, some measures are presented for solving these problems. An optimal voltage regulation method is proposed for medium and low voltage (from 10kV to 0.4kV) distribution network. The regulating frequencies of no-load tap changers and capacitor banks are properly limited. Operating conditions are selected to cover load variat
reactive toions during a long period of time. The combinatorial optimization problem about tap-changers and shunt capacitors is solved by tabu search technology to get the global minima or acceptable results in reasonable time. The simulation results of a practical network in Jinan, China, show that the method is effective in improving the voltage profiles.
VOLTAGE AND REAVTIVE POWER CONDITION IN JINAN POWER SYSTEM
Jinan city is the capital of Shandong province in east China. With increasing rapidly in recent years, the biggest electric power load is about 2200MW in 2004.  The electric power is mainly supply from three power plants and one 500kV substation. There are 11 220kV substations, 24 110kV substations. More than 400 10kV lines supply about 1500 10kV/0.4kV power distribution transformers in the urban district.
With social and economic development in Jinan city, the load difference between peak and valley time becomes so large that the voltage regulation is not enough to meet all the operating conditions.  There exist usually lower voltage in peak time and higher voltage in valley time in the power transmission and distribution networks. The main reasons are unreasonable network structure, large load change ranges, lack of reactive power compensation and voltage regulations.Based on engineering practice a
nd current technology, some measures are outlined for solving these problems. They are paying attention to the reactive power planning, building the reactive power management network, enforcing the power transmission network, keeping the voltage regulating equipments in good condition, improving the distribution network, adding reactive power compensation and voltage regulation devices, paying attention to voltage measurement, improving local reactive power compensation for large users, enforcing harmonic management and keeping high available rate of capacitors. With these measures being taken, the voltage qualified rate is improved and the active power loss is decreased. However, it is need to research in depth the reactive power compensation and voltage management in medium and low voltage distribution networks to ensure the voltage quality of 0.4kV end users.
VOLTAGE REGULATION METHOD
Voltage regulation and/or reactive power optimization aims to improve voltage profiles and reduce active power losses by regulating reactive power flow distribution. Currently, little or no generator is installed in medium and low voltage distribution systems in Jinan, China. Therefore, the regulation means in distribution systems are mainly transformer tap-changers and shunt capacitors, which are discrete variables.
Voltage regulation and/or reactive power optimization in distribution system in this paper is to form a regulation scheme for locations and sizes of capacitors and positions of transformer tap-changers to improve voltage profiles and minimize active power loss and. Therefore, it is a constrained combinatorial optimization problem.
In most of the papers on voltage regulation and/or reactive power optimization in 10kV or higher voltage distribution networks [1-5], on-load tap-changers and shunt capacitors can be adjusted several times a day according to load variations. However, most of the transformers are of no-load tap changers and fixed shunt capacitors cannot be adjusted
CIRED2005Session No 3
automatically in the medium and low voltage (10kV-0.4V)distribution network in Jinan, China. Therefore, a medium voltage (10kV) distribution feeder based voltage regulation method aimed to improve secondary side voltage (0.4kV)profiles is presented in this paper. The regulating frequency of tap-changers and shunt capacitors is limited to several times a year by considering the variation of loads during a long period of time.
Voltage Profiles Assessment
A trapezoid membership function is introduced to evaluate the network voltage profiles. The two main aims of var/voltage optimization, decreasing active power loss and
improving voltage regulation, may conflict. Generally,
reactive power optimization trends to minimize active power loss by increasing voltage to upper voltage limit, which is not acceptable in practice considering variations of loads and source node voltage. If the voltage constraints are treated as “hard” constraints, that is, no voltage violation is allowed; it may be hard to find a feasible solution, especially considering two or more operating conditions. Thus, the voltage profiles constraints are considered as an objective by introducing a membership function to evaluate voltage eligibility of each node and voltage profiles. The membership function for each node voltage is described as
01100
01
1011      1                      0                      upper upper upper i i upper upper
lower lower upper i i lower lower volt i lower upper i V L  L V L L L V L L V L F V L L L V L (others)
­  ° °
° °  ® °
°d d °
°¯  (1)Where, F volt (V i ) is the voltage eligibility membership function for node  i ,  L 0upper  and L 0lower  are unacceptable voltage limits,L 1upper  and L 1lower  are acceptable voltage margins, respectively.The value for voltage profiles assessment is the mean of the membership values of all nodes.Active Power Loss Assessment
Different from the membership function for voltage profiles evaluation, there is not a standard or limit for active power loss [4]. The active power losses in a radiating distribution network will decrease if all reactive power loads are fully compensated and tap-changers adjusted to their upper voltage limits. In such an imagined operating condition, power flow is calculated and the active power loss, P l_min , is considered as the minimum power loss. Each active power loss of trial
solutions, P l , is assessed by the following membership
function
_min _min _min _min _min 0                                  2  22()1                                            l l_ori l l l ori loss
l_l l_ori l l l ori
l l_P P P P P F
P P P P P P P P
­! ° ° d d  ® °° ¯
(2)Where F loss  is the membership function for active power loss,
P l-ori is the active power loss before optimization compensation and regulation,  P l_min  is the minimum active power loss in some imagined operating condition.Voltage Imbalance Assessment
Voltage imbalance is common in distribution networks because of the unbalanced impedances, single-phase loads,phase-to-phase loads and unbalanced three-phase loads.According to the symmetrical component theory, an unbalanced system can be decoupled into three balanced systems, that is, posit
ive sequence, negative sequence and zero sequence system.
As a part of power quality, the unbalance factor for each load should not be bigger than 1.3% in China. The unbalance factor for three-phase line-to-line voltage readings is defined by the IEC as
H  and
22224
44ca
bc
ab
ca bc ab V
V V
V V V
E                      (4)
Where İ is the unbalance factor, V ab , V bc  and V ca  are the line-to-line voltages, respectively.
The membership function for the İ is defined as
10
1010
01                      0              i unbal i i i İİx F İİİİİH H H H ­d °
°  ® °°t ¯
(5)
Where İi  is the unbalance factor of node i , İ0 is an
unacceptable unbalance factor limit and İ1 is an acceptable unbalance factor margin.Capacitor Placement
CIRED2005Session No 3
Properly placed shunt capacitors can improve power factor and voltage quality, and reduce reactive power demand so that active power loss. The nodes with violated voltage-unbalance limits or much varying reactive power demands are considered as to compensate with automatic-switched capacitors (ASC). While fixed shunt capacitors (FC) are placed according to the following sensitivity coefficient of active power loss to reactive power injection under the heaviest load condition                            _loss
i inject i
P SC Q w
w                            (6)
Where SC i  is the sensitivity coefficient, P loss  is the active power loss of the branch, Q inject_i  is the reactive power injection at node i .
The nodes with highest sensitivity coefficient are pre-selected as the nodes to be compensated.Objective Function
The objective function including voltage profiles, active power loss and voltage unbalance under operation condition m  is represented as
_  obj m volt loss unbal m F F F F D E J        (7)
Where F obj_m  is the objective function for operating condition m , Į, ȕ and Ȗ are weight factors for the three objectives.Thus, the voltage regulation problem is described as the following
_1
min max
max
Max
..                0M
obj obj m m k k k cj cj F F s t T T T Q Q  d d d d ¦              (8)
Where  M  is the number of operating conditions, T k  is the ratio of transformer k , Q cj  is the reactive power of capacitors at node j . The restriction of power flow is not listed here.Optimization Algorithm
To get the global minima or acceptable results in reasonable time, above-mentioned combinatorial opti
mization problem can be solved by a genetic algorithm or tabu search
technology. The process of the tabu search adopted is described as following.i)
Read in initial data, including impedance of feeders,loads, regulation variables and inequality constraint conditions. Code the regulation variables.
ii) Generate initial solution. Set the regulation variables
randomly without breaking the constraint conditions,including power flow restriction. Calculate the objective function f(X), and set best solution vector X opt  as X which is consisting of T k  and  Q cj .iii) Generate a group of trail solutions, X 1, X 2, … , X k , by
“move” from X , and calculate the responding values of objective function, f(X 1), f(X 2), …, f(X k ).iv) Search neighborhoods. Get the best one, X* , from the
trail solutions. Update X  with X*, if X* is not in the Tabu list, or X* fits the aspiration criteria. Try the next solution, if the former one cannot update X .v) Update Tabu list. Push the record of reversed move into
the Tabu list, which is a FIFO (First-In-First-Out) stack.vi) Update X opt  with X*, if f(X*) is better than f(X opt ).vii) Terminate condition. Stop optimization and output
results if f(X opt ) has not been improved for several iterations  or the  given maximum iteration number is met. Otherwise go to step iii) to continue.Software Package
A Client/Server software package based on the proposed approach is implemented in C++ with man–machine interactive procedures, which has following distinct features. i)  Tabu search optimization technology is utilized in the
software to solve the optimization problem. Compared with the traditional mathematical optimization algorithms,the heuristic searching and optimizing technique can avoid trapping in local optimums and get the global optimum with high probability or acceptable solutions in reasonable time.ii)  The package employs two kinds of databases. Customer
database is used to store the data of various customer networks, including node data, line data and transformer data. Common database is used to store the information of common devices, including line common data and transformer common data. The data of customer database can be inputted by users or selected from common database directly. Users can get data via ODBC from the SCADA syst
em, which enhances the online optimization ability of the software package.
CIRED2005Session No 3
SIMULATION RESULTS
The effectiveness of the proposed method is verified by the application to a practical distribution feeder shown as Fig.1 in Jinan, China. Data are sampled by the SCADA system every
15 minutes.
Fig.1 Distribution network structure
Three-phase power flow [5] is calculated by a forward-backward sweeping algorithm. All transformers in this network have no-load tap-changers (NLTC), which are of 5positions (1±2.5%×2). All the original positions are 3, that is,all the ratios are 1 p.u. The phase-to-neutral voltage upper and lower limits for 220V distribution networks are 7% (235.4V)and -10% (198V) respectively.
The node (6000#) with most unbalanced reactive power demands is selected to compensate with nonsymmetrical auto-switched capacitors. So the reactive power loads at this node are almost fully compensated.
Some of optimization results are given as following. Tap changers of all the transformers are regulated from position 3to position 1, that is, to decrease the ratios from 1 to 0.95 in p.u. Three transformers with biggest sensitivity values are selected to be compensated. They are node 6010#, 6007# and 6008#. The compensation capacities are 18, 12 and 18 kVar,respectively. Transformer 6010# is of the smallest VA capacity (250kVA), whose impedance is bigger than that of the others (315kVA or 400kVA).  After optimization, the voltage profiles are improved quite effectively with small active power loss decrease. In other words, all the voltages at different conditions are in the permitted range.
CONCLUSIONS
The main reasons of voltage and reactive power problems in Jinan power distribution system are outlined. Proper measures are presented based on engineering practice and current technology.
An optimal voltage regulation method is proposed for medium and low voltage distribution networks. The reactive power optimization problem is formed as a multi-object optimization problem, which aims to improve voltage profiles, decrease active power losses and balance voltage imbalance. Membership functions are introduced to balance different objectives. The regulating frequencies of no-load tap-changers and capacitor banks are properly limited.Operating conditions are selected to cover load variations during a long period of time. The combinatorial optimization problem is solved by tabu search technology to get the global minima or acceptable results in reasonable time. The simulation results of a practical network show that the method is effective in improving the voltage profiles.
REFERENCES
[1] Y. Liu, et al. 2002, “Optimal voltage/var control in distribution systems”, Int J Elec Power, vol.24, 271-276.[2] Z. Hu, et al. 2003, “V olt/Var control in distribution systems using a time-interval based approach”, IEE P-Gener Transm D, vol.150, 548-554.
[3] S. F. Mekhamer, et al. 2003, “Application of fuzzy logic for reactive-power compensation of radial distribution feeders” IEEE T Power Sys, vol.18, 206-213.
[4] C. Su, et al. 1996, “A new fuzzy-reasoning approach to optimum capacitor allocation for primary distribution systems ”, Proceedings Industrial Technology, IEEE pp.237-241.
[5] W. Lin, et al. 1999, “Three-phase unbalanced distribution power flow solutions with minimum data preparation”,IEEE T Power Sys, vol.14, 1178-1183.

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